GitLab

This CL:
1- Replaces usage of didMoveTab with didMoveWebState in TabStripController
   and in BrowserViewController
2- Create FakeWebStateListObservingDelegate to be used in tests that
   requires the Obj-C version of WebStateListObserver.
3- Update tab_model_unittest to use the FakeWebStateListObservingDelegate
   for testing.

Change-Id: I8868442244ad3e2933823d3b6f5e1c5fe047db9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614030Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Mohammad Refaat <mrefaat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660813}